Cation-Exchange-Induced Enhanced Capacitive Performance of Ternary ZnCoNi Carbonate Hydroxides for High-Energy-Density Hybrid Supercapacitors.

The development of high-performance electrode materials is critical for advancing the energy density of hybrid supercapacitors. Herein, a cation exchange strategy is proposed to optimize the electrochemical properties of a ternary ZnCoNi carbonate hydroxide (ZnCoNi-CH) electrode. The selective Ni2+-for-Zn2+ cation exchange in an ethanol solution effectively preserved the original morphology of the ZnCoNi-CH nanoarray while enhancing structural defects and synergistic bimetallic effects. The cation-exchanged electrode (ZnCoNi-CH-ex) exhibited a significantly increased areal capacity of 3.80 C cm-2 at 10 mA cm-2, surpassing that of the pristine ZnCoNi-CH electrode (2.55 C cm-2). XPS and EPR analyses revealed that cation exchange modulated the valence states of Ni3+/Ni2+ and Co3+/Co2+, promoting electron transfer and surface redox activity. Structural defects introduced during the exchange process created high-index crystal planes, facilitating electrolyte penetration and charge transport. As a result, the ZnCoNi-CH-ex electrode demonstrated superior rate capability (60.7% capacity retention at 100 mA cm-2) and exceptional cycling stability (94.7% capacity retention after 12,000 cycles at 50 mA cm-2). When integrated into an all-solid-state hybrid supercapacitor with activated carbon, the device delivered a high energy density of 1.0 mWh cm-2 at a power density of 9 mW cm-2, maintaining 105.9% capacity retention after 15,000 cycles. Theoretical calculations confirmed that cation exchange narrowed the bandgap and reduced OH- adsorption energy, synergistically enhancing conductivity and reaction kinetics. This work establishes an effective strategy for tuning the electronic structure and interfacial properties of electrode materials for high-performance hybrid supercapacitors.